The initial parameters for the reactant (syn-CH\(_3\) CHOO,
methyl-substituted Criegee intermediate) and product (CH\(_2\) CHOOH,
vinyl hydroperoxide) were taken from
SwissParam.[1]} First, representative structures
for CH\(_3\) CHOO (1612) and CH\(_2\) CHOOH (1650) were sampled from 500 ps
MD simulations at 300 K. For these structures, energies were
determined at the MP2/6-311++G(2d,2p) level of theory. These were fit
separately to parametrized force fields for the reactant and the
product using a downhill simplex[2]}
algorithm. Parametrization starts with a set of 100 structures for
CH\(_3\) CHOO and CH\(_2\) CHOOH and first fitting for the two states were
carried out which was then followed by further MD simulations using
this improved set of parameters from which another 200 structures was
extracted and included in the fit. Several rounds of refinements were
done until the root mean squared deviation for the final set between
the target (ab initio) and the fitted energies for CH\(_3\) CHOO
and CH\(_2\) CHOOH reached 1.1 kcal/mol and 1.2 kcal/mol,
respectively. The bonds involved in bond breaking and bond formation
(the three C-H, the O-O, and the O-H bonds) were described by Morse
potentials and the charges from ab initio calculations at the
MP2/6-311++G(2d,2p) level of theory. Generalized Lennard-Jones
potential is included and fitted, to represent the van der Waals
interaction between atoms(between H\(_{\rm A}\)  and O\(_{\rm B}\)  in
reactant FF and between H\(_{\rm A}\)  and C\(_ {\rm A}\)  in product FF) in
the reactive region.
